I. Field of the Invention
The present invention relates generally to the field of wireless communications. More particularly, the present invention relates to resolving frequency and timing uncertainty in access channel transmissions in a spread spectrum communication system.
II. Related Art
Typical wireless satellite-based communications systems include base stations referred to as gateways, and one or more satellites to relay communications signals between the gateways and one or more user terminals. Gateways provide communication links for connecting a user terminal to other user terminals or users of other communications systems, such as a public telephone switching network. User terminals can be fixed or mobile, such as a mobile or portable telephone. They may be located near or remote from a gateway.
Some satellite communications systems employ code division multiple access (CDMA) spread-spectrum signals, such as disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled “Spread Spectrum Multiple Access Communication System Using Satellite or Terrestrial Repeaters,” and U.S. Pat. No. 5,691,974, which issued Nov. 25, 1998, entitled “Method and Apparatus for Using Full Spectrum Transmitted Power in a Spread Spectrum Communication System for Tracking Individual Recipient Phase Time and Energy,” both of which are assigned to the assignee of the present invention, and are incorporated herein by reference.
In satellite communication systems employing CDMA, separate communication links are used to transmit communication signals, including paging, access, messaging, or traffic signals, to and from a gateway or base station. A forward communication link refers to communication signals originating at a gateway or base station and transmitted to a user terminal. A reverse communication link refers to communication signals originating at a user terminal and transmitted to a gateway or base station.
The reverse link is comprised of at least two separate channels: an access channel and a reverse traffic channel. The access channel is used by a user terminal to “access” a gateway. A user terminal accesses a gateway to register with the system, to place a call, or to acknowledge a paging request sent by the gateway. A user terminal communicates with a gateway on the access channel by transmitting a signal referred to as an “access probe” to the gateway. An access probe is a transmission of data on the access channel that contains an access message. The contents of the access message depend on whether the user terminal is initiating a call, registering with the system, or responding to a page.
In a typical spread spectrum communications system, one or more preselected pseudo noise (PN) code sequences are used to “spread” information signals, such as an access probe, over a predetermined spectral band prior to modulation onto a carrier signal for transmission as communications signals. PN code spreading, a method of spread spectrum transmission that is well known in the art, produces a signal for transmission that has a bandwidth much greater than that of the data signal.
In order for a gateway to acquire an access probe sent by a user terminal (i.e., recover the access message within the access probe), the gateway must first demodulate the communication signal to recover the PN modulated access probe, and then despread the message portion of the access probe. In order for the gateway to demodulate the carrier, the gateway must be tuned to the carrier frequency of the communication signal. Without reasonably accurate frequency tuning, the carrier cannot be properly demodulated. Furthermore, because PN spreading codes are applied to the access probe, the arrival time of the access probe must be determined to properly despread the access probe to recover the information contained therein. PN spreading codes cannot be accurately removed without appropriate system timing or signal synchronization. If the codes are applied with incorrect time synchronization, the communication signals will simply appear as noise and no information is conveyed.
Communication systems employing satellites with non-geostationary orbits exhibit a high degree of relative user terminal and satellite motion. The relative motion creates fairly substantial Doppler components or shifts in the carrier frequency of signals within the communication links. Because these Doppler components vary with user terminal and satellite motion, they create a range of uncertainty in the frequency of the carrier signal, or more simply, frequency uncertainty. Similar effects may be observed in terrestrial systems where the user terminal is moving at a high speed, such as when used on a high speed train or other vehicle.
The satellite motion also introduces Doppler into the PN spreading codes. This Doppler is referred to as code Doppler. In particular, code Doppler is the effect of the satellite motion introduced into the baseband signal. Code Doppler shifts the frequency of the transitions between adjacent codes in the PN spreading code sequences. Thus, the adjacent codes do not arrive at the receiver with a correct code timing.
In addition to code Doppler, the satellite motion also creates a large amount of uncertainty in the propagation delay, or timing uncertainty, for signals within the communication links. For signals arriving at the gateway, the propagation delay varies from a minimum when the satellite is directly overhead of the gateway to a maximum when the satellite is at a horizon with respect to the gateway.
As stated above, in order for the gateway to acquire an access probe, the gateway must be tuned to the carrier frequency of the communication signal and synchronize timing with the signal. One way to tune the gateway to the carrier frequency and synchronize timing is to determine the carrier frequency and timing prior to the transmission of the communication signal and then tune the gateway appropriately. But because of the frequency and time uncertainty introduced into the communication signal by the Doppler effect and propagation delay, a gateway cannot determine the carrier frequency or signal arrival time prior to receiving the signal. Nevertheless, the gateway can determine the range of possible carrier frequencies and the range of possible arrival times by determining the amount of uncertainty introduced by the Doppler effect and propagation delay. Consequently, a gateway can acquire an access probe by “searching” for the correct frequency and timing by comparing the received communication signal with various frequency and timing values within their respective possible ranges.
These various frequency and timing values are termed frequency and timing hypotheses, respectively. The frequency and time hypothesis with the highest correlation to the received communication signal above a predetermined threshold provides frequency and timing values that can be used to demodulate and despread the signal, thereby enabling the gateway to recover the information within the access probe.
The amount of hardware that is required to “search” for the correct frequency and timing in a fixed amount of time is proportional to the number of required hypotheses, and the number of required hypotheses is a function of the range of time and frequency uncertainty. Because searcher hardware is expensive and because it is undesirable to increase the search time, a system and method to reduce the range of time and frequency uncertainty is therefore desired.